A rap or tumble upsets one out for a stroll (12)

I believe the answer is:

perambulator

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'a stroll' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

'a rap or tumble upsets one out' is the wordplay.
'upsets' is an anagram indicator.
'one out' means one lot of letters goes inside another.
'rap'+'or'+'tumble'='raportumble'
'raportumble' is an anagram of 'perambultor'.
'a' put inside 'perambultor' is 'PERAMBULATOR'.

'for' acts as a link.

Can you help me to learn more?

(Other definitions for perambulator that I've seen before include "Baby carriage" , "Baby's transport" , "Baby carrier (usually abbreviated)" .)

I've seen this clue in The Globe and Mail.

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